In recent years, hydrogen fuel has been attracting attention as a substitute energy for fossil fuels such as petroleum, because it is a renewable and clean energy which, unlike fossil fuels, emits little carbon dioxide and other environmental pollutants by burning. For this reason, more efficient hydrogen production methods have been enthusiastically studied all over the world.
Hydrogen can be produced from various production sources, although it is preferably produced from biomass as a production source from a viewpoint of recycling. Such hydrogen production methods from biomass mainly involve thermochemical methods and biological methods with use of bacteria. Of these, biological methods are preferred. The reason is that thermochemical methods such as hot gasification are cost-consuming since biomass normally contains lots of moisture because of its nature as an energy product of organic wastes, sugar cane, etc.
Bacteria for use in hydrogen production from biomass can be largely categorized in photosynthetic bacteria and fermentative bacteria. The photosynthetic bacteria are capable of complete decomposition of organic matters contained in biomass into water and carbonic gas by using light energy. However, the drawback is that light energy can be used only during the day and becomes insufficient in the morning and the evening. On the other hand, the fermentative bacteria are capable of producing hydrogen even in a sealed container, and are suitable for hydrogen production from biomass having high moisture contents such as raw garbage and waste molasses. In addition, the hydrogen production rate of fermentative bacteria is higher than that of photosynthetic bacteria. For these reasons, hydrogen production methods with use of fermentative bacteria have been mainly employed in the field of industry.
Anaerobic fermentative bacteria for use in the hydrogen production from biomass include bacteria belonging to the genus Clostridium such as Clostridium butyricum and bacteria belonging to the genus Enterobacter such as Enterobacter aerogenes (for example, refer to Non-patent Document 1). As a hydrogen production method from biomass with use of a bacterium belonging to the genus Clostridium, for example, there is disclosed (1) a method for production of hydrogen by generating hydrogen from an organic material, comprising: a charging step for charging the organic material; a microorganism-charging step for charging a microorganism belonging to the genus Clostridium; a reaction step for reacting the microorganism with the organic material to produce hydrogen; and a stirring step, during the reaction step, for stirring the organic material and the microorganism in order to promote the reaction and decomposition (for example, refer to Patent Document 1). In addition, as a hydrogen production method from biomass with use of a bacterium belonging to the genus Enterobacter, for example, there is disclosed (2) a method for producing hydrogen and ethanol from a raw material liquid which contains a biodiesel waste liquid obtained through demethylesterification of a methylesterified-oil and fat wherein the method comprises a fermentation step for fermenting with a bacterium belonging to the genus Enterobacter at least in the presence of a carrier whose surface is capable of immobilizing microorganisms (for example, refer to Patent Document 2).
Bacteria belonging to the genus Clostridium and bacteria belonging to the genus Enterobacter are normally hydrogen-producing bacteria which produce hydrogen at 30 to 38° C. In contrast, there exist high-temperature hydrogen-producing bacteria which produce hydrogen at 65 to 80° C., and high-temperature hydrogen fermentation methods using the same. As compared to the hydrogen production methods using bacteria belonging to the genus Clostridium or the like, the high-temperature hydrogen fermentation methods are advantageous in the points of improved hydrogen yield and easy prevention of the substrate contamination caused by various germs. For example, Thermotoga maritima has achieved a theoretically maximum hydrogen yield of 4 mol/mol-glucose by batch cultivation at 80° C. (for example, refer to Non-patent Document 2). Such a high hydrogen yield at high temperature can be attributed to the suppression against metabolism of undesirable by-products such as lactic acid.
Non-patent Document 1: de Vrije and Classen (2003) “Dark hydrogen fermentations” in Bio-methane and Bio-hydrogen, p103 to 121
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2001-157595
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2006-180782
Non-patent Document 2: Schroder et al. (1994) Archives of microbiology 161: p460 to 470